Modeling the properties of open d-shell molecules with a multi-determinantal DFT

La théorie du champ des ligands a été utilisée avec succès durant des décennies pour décrire l’état fondamental et les états excités des complexes. Les chimistes utilisent cette théorie afin d’interpréter des spectres UV-Vis essentiellement. D’un autre côté, les chimistes computationels peuvent décrire assez précisément les propriétés correspondant à l’état fondamental mais les modèles permettant de décrire les propriétés des états excités sont encore en voix de développement. La méthode "Champ des Ligands - Théorie de la Fonctionnelle de la Densité", qui est la méthode présentée dans cette thèse, propose un lien entre la Théorie de la Fonctionnelle de la Densité (TFD) appliquée à l’état fondamental et la modélisation des propriétés des états excités par l’intermédiaire de la théorie du champ des ligands. Pour ce faire, nous calculons grâce à la TFD les énergies de tous les déterminants de Slater due à une configuration dn en référence a un état correspondant à une configuration moyenne (répartition égale des électrons d dans les 5 orbitales moléculaires correspondant aux orbitales d de l’élément de transition) afin de satisfaire aux exigences de la théorie du champ des ligands. Dans un premier temps, la méthode est appliquée à des composés connus afin de tester sa validité. Dans un deuxième temps, le champ d’application de la méthode est étendue à la modélisation des tenseurs g et A. Tout au long de cette thèse, les résultats obtenus sont comparés aux données expérimentales obtenues par les chimistes. Nous montrons aussi que la méthode donne plus d’informations que l’on ne pouvait espérer, en particuliers, lors du traitement des effets relativistes.Ligand field theory has been used along decades with success to describe ground and excited electronic states originating from dn transition metals complexes. Experimental chemists use such a theory to interpret spectra. On the opposite side, computational chemists can describe with good accuracy the ground states properties but models to calculate excited states properties are still being developed. The Ligand Field –Density Functional Theory, which is the method presented in this thesis, proposes a link between the density functional theory applied to ground state and the determination of excited states properties through the ligand field theory. To achieve this, we compute within the DFT formalism the energies of all the Slater determinants originating of a dn configuration taken as reference an average of configuration to satisfy the requirement of the Ligand Field Theory. In a first step, the method is applied to well known compounds to test the ligand field and Racah’s parameterization in comparison to values fitted from experimental UV-Vis spectra. Then we use a Ligand field program to predict the multiplet structure. Next, extension of the method is proposed to determine ESR parameters and relativistic effect within the same formalism. At each step, the results are compared to data which are well known for many decades by the chemists. We will also show the ability of the method to give more informations than usually expected

The Impact of Giulio Racah on Crystal- and Ligand-field Theories

This paper focuses on the impact of Racah on crystal- and ligand-field theories, two branches of molecular physics and condensed matter physics (dealing with ions embedded in aggregates of finite symmetry). The role of Racah and some of his students in developing a symmetry-adapted weak-field model for crystal-field theory is examined. Then, we discuss the extension of this model to a generalized symmetry-adapted weak-field model for ligand-field theory. Symmetry considerations via the use of the Wigner-Racah algebra for chains of type SU(2) > G is essential for these weak-field models. Therefore, the basic ingredients for the Wigner-Racah algebra of a finite or compact group are reviewed with a special attention paid to the SU(2) group in a SU(2) > G basis. Finally, as an unexpected application of nonstandard SU(2) bases, it is shown how SU(2) bases adapted to the cyclic group allow to build bases of relevance in quantum information

Crystal Field in Rare-Earth Complexes:From Electrostatics to Bonding

The flexibility of first-principles (ab initio) calculations with the SO-CASSCF (complete active space self-consistent field theory with a treatment of the spin-orbit (SO) coupling by state interaction) method is used to quantify the electrostatic and covalent contributions to crystal field parameters. Two types of systems are chosen for illustration: 1)The ionic and experimentally well-characterized PrCl3 crystal; this study permits a revisitation of the partition of contributions proposed in the early days of crystal field theory; and 2)a series of sandwich molecules [Ln(ηn-CnHn)2]q, with Ln=Dy, Ho, Er, and Tm and n=5, 6, and 8, in which the interaction between LnIII and the aromatic ligands is more difficult to describe within an electrostatic approach. It is shown that a model with three layers of charges reproduces the electrostatic field generated by the ligands and that the covalency plays a qualitative role. The one-electron character of crystal field theory is discussed and shown to be valuable, although it is not completely quantitative. This permits a reduction of the many-electron problem to a discussion of the energy of the seven 4f orbitals

Point Defects in Lithium Gallate and Gallium Oxide

Electron paramagnetic resonance (EPR), Fourier-Transform Infrared spectroscopy (FTIR), photoluminescence (PL), thermoluminescence (TL), and wavelength-dependent TL are used to identify and characterize point defects in lithium gallate and β-gallium oxide doped with Mg and Fe acceptor impurities single crystals. EPR investigations of LiGaO2 identify fundamental intrinsic cation defects lithium (V−Li) and gallium (V2−Ga) vacancies. The defects’ principle g values are found through angular dependence studies and atomic-scale models for these new defects are proposed. Thermoluminescence measurements estimate the activation energy of lithium vacancies at Ea = 1.05 eV and gallium vacancies at Ea \u3e 2 eV below the conduction band minimum. Mg and Fe doped β-Ga2O3 crystals are investigated with EPR and FTIR and concentrations of Ir4+ ions greater than 1 × 1018 cm3 are observed. The source of the unintentional deep iridium donors is the crucible used to grow the crystal. In the Mg-doped crystals, the Ir4+ ions provide compensation for the singly ionized Mg acceptors contributing to the difficulties in producing p-type behavior in bulk single crystals. A large spin-orbit coupling causes Ir4+ ions to have a low-spin (5d5, S = 1/2) ground state. The Ir4+ ions have an infrared absorption band representing a d − d transition within the t2g orbitals. Using these same techniques the Fe2+/3+ level in Fe-doped β-Ga2O3 crystals is determined. With these noncontact spectroscopy methods, a value of 0.83 ± 0.04 eV below the conduction band is obtained for this level. These results clearly establish that the E2 deep level observed in DLTS experiments is due to the thermal release of electrons from Fe2+ ions

High-field electron spin resonance study of electronic inhomogeneities in correlated transition metal compounds

Electronic inhomogeneities play an important role in the definition of physical properties of correlated systems. To study these inhomogeneities one has to use local probe techniques which can distinguish electronic, magnetic and structural variations at the nanoscale. In the present work the high-field electron spin resonance technique (HF-ESR) is used to probe electronic and magnetic inhomogeneities in two transition-metal element based systems with very different properties. The first system is an iron based hightemperature superconductor, namely a member of a so called 1111-family, the (La,Gd)O1−xFxFeAs compound. Our HF-ESR spectroscopy study on Gd3+ ion has revealed that this material exhibits anisotropic interaction between Gd and Fe layers, which is frustrated in the absence of an external magnetic field. Moreover, the study of the superconducting samples has shown a coexistence of a static short range magnetic order with superconductivity up to high doping levels. The second system is a lightly hole doped cubic perovskite LaCoO3. Here, our HF-ESR investigation, complemented with static magnetometry and nuclear magnetic resonance techniques, has established that the hole doping induces a strong interaction between electrons on neighboring Co ions which leads to a collective high-spin state, called a spin-state polaron. These polarons are inhomogeneously distributed in the nonmagnetic matrix. This thesis is organized in three chapters. The first chapter gives basic ideas of magnetism in solids, focusing on the localized picture. The aim of the second chapter is to introduce the method of ESR. The third chapter is dedicated to the study of 1111-type iron arsenide superconductors. In the first part X-band (9.5 GHz) ESR measurements on 2% and 5% Gd-doped LaO1−xFxFeAs are presented. In the second part a combined investigation of the properties of GdO1−xFxFeAs samples by means of thermodynamic, transport and high-field electron spin resonance methods is presented. The last, fourth chapter presents the investigation of the unexpected magnetic properties of lightly hole-doped LaCoO3 cobaltite by means of the electron spin resonance technique complemented by magnetization and nuclear magnetic resonance measurements

Tunable High-Field/ High-Frequency ESR and High-Field Magnetization on Single-Molecule Clusters

In this work, low dimensional iron group clusters have been studied by application of high magnetic fields. The magnetization has been probed with an MPMS as function of temperature and field. The combination with pulse field measurements up to 52\,T allowed determination of the magnetic exchange coupling parameters, and to probing the effective spin of the ground state. The main focus was on tunable high-field/high-frequency (tHF) ESR in static fields < 17 T and pulse field ESR up to 36 T. This magnetic resonance method has been used for the characterization of the local magnetic properties: The detailed analysis of the field dependence of dedicated spin states allowed to determine the magnetic anisotropy and g-factors. The results were analyzed in the framework of the appropriate effective spin Hamiltonians in terms of magnetization fits and ESR spectrum simulations

Tunable High-Field/ High-Frequency ESR and High-Field Magnetization on Single-Molecule Clusters

In this work, low dimensional iron group clusters have been studied by application of high magnetic fields. The magnetization has been probed with an MPMS as function of temperature and field. The combination with pulse field measurements up to 52\,T allowed determination of the magnetic exchange coupling parameters, and to probing the effective spin of the ground state. The main focus was on tunable high-field/high-frequency (tHF) ESR in static fields < 17 T and pulse field ESR up to 36 T. This magnetic resonance method has been used for the characterization of the local magnetic properties: The detailed analysis of the field dependence of dedicated spin states allowed to determine the magnetic anisotropy and g-factors. The results were analyzed in the framework of the appropriate effective spin Hamiltonians in terms of magnetization fits and ESR spectrum simulations